1. Field of the Invention
The invention relates to electroless deposition of metals. More particularly, it relates to the selective deposition of nickel on aluminum metallized semiconductor devices in predetermined areas defined by openings in a suitable dielectric or photoresist. Electroless deposition of gold is also described.
2. Description of the Prior Art
Aluminum is one of the preferred metals for semiconductor active device contacts for various reasons such as ease of evaporation, good electrical conductivity, and lack of adverse side effects on the electrical characteristics of the devices. However, the use of aluminum has two major problems: (1) it is not directly solderable and (2) it rapidly forms an impervious oxide. It is, thus, difficult to bond wire leads to the aluminum contact. One solution is direct thermocompression bonding of gold to aluminum. However, the composite degrades into a brittle intermetallic which degrades the contact. Another current technique is multimetallization as used for beam lead fabrication. This is a complex and costly procedure involving multiple photolithography steps to apply Ti/Pd/Au or Ti/Pt/Au metallization.
Nickel is an inexpensive, solderable material which can be used on top of aluminum metallization to enable contact of leads to the aluminum. Nickel also has the advantages of being harder than aluminum and more corrosion resistant. However, the formation of aluminum oxide has made it difficult to deposit nickel directly on aluminum without extensive pretreatment. A common pretreatment technique is zincating, the deposition of an intermediate zinc film which replaces the aluminum/aluminum oxide. Another example is ion activation, the activation of the surface with tin or palladium ions. Fluoride ions have also been used for activation but in large concentrations will etch into the aluminum. Ion activation and zincating overactivate and can cause deposition of nickel in areas other than where desired e.g., on a dielectric mask. The metals deposited during pretreatment also diffuse into the aluminum. Zinc diffusion, for example, reduces device lifetime by causing the aluminum to become brittle and, in the case of silicon, by altering the doping level.